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Actually, apparently it’s Lucas’ gift for Athy’s 15th birthday.
Spoiler(?) ahead(?)
So he disappears for a year to find the world tree, finds it, comes back, finds out why Athy is sad (dad forgot her), teleports himself and Athy to her father, sneaks up and wacks/stabs her father’s head with a branch of the world tree, bibbidy-bobbidy-boo he remembers everything. Supposedly he even remembers the original life in the novel.
Found this info on a spoilers thread in novelupdates. Apologies for inaccuracies.

Claude will remember athy after a year. And it's her 15th birthday. Aww for the first time he laugh and pat athy back. Athy was crying, she can't believed it. She hug her dad tightly afraid that he'll gone away again.